Polishing composition and magnetic recording disk substrate polished with the polishing composition

ABSTRACT

A polishing composition includes at least water, alumina and a sol product derived from an aluminum salt. A magnetic recording disk substrate polished with the polishing composition suppresses formation of roll-off on the outer peripheral portion thereof, has a high-quality mirror-finished surface with few pits, nodules and scratches, and enables a distance between it and a magnetic head to be small, thereby making it possible to the recording density.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is an application filed under 35 U.S.C. §111(a)claiming the benefit pursuant to 35 U.S.C. §119(e) (1) of the filingdate of Provisional Application No. 60/260,883 filed Jan. 12, 2001pursuant to 35 U.S.C. §111(b), and is a Continuation-in-Part applicationof pending PCT/JP01/05800, filed Jul. 4, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a polishing composition and,more particularly, to a composition for polishing a substrate of amagnetic recording disk to be incorporated into a rigid disk driveincluded in a computer and to a method of producing a magnetic recordingdisk substrate polished with the polishing composition.

[0004] 2. Description of the Prior Art

[0005] Conventionally, there have been proposed a variety ofcompositions for polishing a magnetic recording disk substrate thatexhibit a high polishing rate and provide, on the surface of asubstrate, few defects such as scratches, pits and nodules, and littlepolishing-induced damage. For example, JP-A-SHO 61-291674 discloses apolishing composition containing sulfamic acid or phosphoric acid,JP-A-SHO 62-25187 a polishing composition containing aluminum nitrate,and JP-A-HEI 2-158682 a polishing composition containing a metalnitrite.

[0006] JP-A-HEI 4-275387 discloses a polishing composition containingtwo types of polishing accelerators, one of which is aluminum sulfate oraluminum chloride and the other of which is a peroxide, nitric acid, anitrate salt, a nitrite salt or an aromatic nitro compound.

[0007] In addition, polishing compositions containing boehmite, boehmitealumina sol or colloidal alumina as a polishing accelerator, aredisclosed as effective compositions for yielding a high-quality polishedsurface without generating surface defects.

[0008] Other polishing compositions are reported, for example, inJP-A-HEI 1-188264 (a composition containing alumina added withboehmite), JP-A-HEI 1-205973 (a composition containing alumina addedwith a metal salt and boehmite), JP-A-HEI 2-84485 (a polishingcomposition comprising gluconic acid, lactic acid, sodium salts thereofand colloidal alumina), JP-A-HEI 2-158683 (a composition containingalumina added with boehmite and an ammonium salt of an inorganic ororganic acid), JP-A-HEI 3-115383 (a composition containing alumina addedwith boehmite and a water-soluble peroxide), JP-A-HEI 4-363385 (acomposition containing alumina added with a chelate compound, boehmite,and an aluminum salt), and JP-A-HEI 11-92749 (a composition comprisingalumina, boehmite and a polyamine chelate compound or apolyaminocarboxylic acid chelate compound).

[0009] These compositions have been developed so as to provide ahigh-quality polished surface having no surface defects such as pits,nodules and scratches, while maintaining a high polishing rate.

[0010] However, in the rapidly growing field of computer hardware, sincethe narrower the distance (flying height) between a magnetic head and amagnetic recording disk in a rigid disk drive, the higher the recordingdensity, a ceaseless demand is posed for a magnetic recording diskhaving a higher-quality finished surface. However, hitherto, no magneticrecording disk fully satisfying such a ceaseless demand for practicalperformance has yet been produced.

[0011] In order to enhance the recording density as described above, thedisk substrate must have a high degree of flatness; low surfaceroughness; and no pits, nodules or scratches, and must have littleroll-off that would possibly be formed on the outer peripheral endportion of a disk. In the case in which a polished surface of highquality having, among other properties, a surface roughness Ra ofapproximately 15 Å or less is demanded, ultramicro-scale pits andnodules, which have conventionally been accepted, pose a problem.Therefore, there is a demand for an excellent polishing composition thatcan provide a finished surface of high quality.

[0012] In order to satisfy these demands, an object of the presentinvention is to provide a polishing composition that can provide ahigh-quality polished surface having no surface defects whilemaintaining a high polishing rate. More particularly, the object is toprovide a polishing composition that, during polishing of a Ni−P-platedaluminum substrate of a magnetic recording disk, attains high polishingefficiency and can form an excellent polished surface having highsmoothness and no surface defects.

SUMMARY OF THE INVENTION

[0013] To attain the above object, the present invention provides apolishing composition comprising at least water, alumina and a solproduct derived from an aluminum salt.

[0014] The polishing composition can further contain a polishingaccelerator.

[0015] The polishing accelerator is at least one species selected fromthe group consisting of organic acids, inorganic acids and saltsthereof.

[0016] The sol product is a mixture of an aluminum salt with at leastone species selected from the group consisting of sodium hydroxide,potassium hydroxide, ammonia, organic amine compounds, amine chelatecompounds, aminocarboxylic acids, aminocarboxylic acid chelate compoundsand aminophosphonic acid chelate compounds.

[0017] The sol product can be a mixture of at least one species selectedfrom among hydrates and anhydrates of aluminum salts including inorganicacid aluminum salts that include aluminum sulfate, aluminum chloride,aluminum nitrate, aluminum phosphate and aluminum borate, and organicacid aluminum salts that include aluminum acetate, aluminum lactate andaluminum stearate with at least one species selected from among sodiumhydroxide, potassium hydroxide, ammonia, organic amine compounds, aminechelate compounds, aminocarboxylic acids, aminocarboxylic acid chelatecompounds and aminophosphonic acid chelate compounds.

[0018] The sol product can be a mixture of at least one aluminum saltselected from the group consisting of aluminum sulfate, aluminumchloride and aluminum nitrate with at least one compound selected fromthe group consisting of sodium hydroxide, potassium hydroxide, ammonia,triethanolamine and aminotrismethylenephosphonic acid.

[0019] The polishing accelerator is contained in an amount of 0.01-10mass %.

[0020] The sol product is contained in an amount of 0.01-5 mass %.

[0021] The present invention further provides a method of producing asol product derived from an aluminum salt, which comprises mixing, bymeans of a stirrer, an aluminum salt with at least one species selectedfrom the group consisting of sodium hydroxide, potassium hydroxide,ammonia, organic amine compounds, amine chelate compounds,aminocarboxylic acids, aminocarboxylic acid chelate compounds andaminophosphonic acid chelate compounds.

[0022] In the method of producing a sol product described above, thealuminum salt is at least one species selected from the group consistingof aluminum sulfate, aluminum chloride and aluminum nitrate.

[0023] In the method of producing a sol product described above, thestirrer is a high-shear stirrer.

[0024] The present invention further provides a method of producing amagnetic recording disk substrate, which comprises rotating at least oneof a magnetic recording disk raw substrate and a polishing pad while anyone of the polishing compositions is fed into a space between thesubstrate and the pad.

[0025] In the method of producing a magnetic recording disk substratedescribed above, the magnetic recording disk raw substrate is analuminum disk raw substrate that is chemically plated with Ni—P.

[0026] According to the polishing composition of the present inventionthat contains a sol product derived from an aluminum salt, it ispossible to obtain a substrate that has suppressed roll-off from beingformed on the outer peripheral end portion of a disk and has amirror-finished surface of high quality with no surface defects whilemaintaining a high polishing rate. When the substrate is used as amagnetic recording disk, it is possible to make the distance between itand a magnetic head narrower, thereby enhancing the recording density.

[0027] The above and other objects, features and advantages will becomeapparent from the description made with reference to the accompanyingdrawings.

BRIEF DESCRIPTON OF THE DRAWINGS

[0028]FIG. 1 is a schematic view showing the constitution of one exampleof a high-shear stirrer used for producing a sol product according tothe present invention.

[0029]FIG. 2 is an explanatory sketch for determining the amount ofroll-off of a polished disk.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0030] The present invention is directed to a polishing compositioncomprising water, alumina and a sol product derived from an aluminumsalt. Preferably, the composition further contains a polishingaccelerator in addition to the above components. More particularly, thepresent invention is directed to a polishing composition into which asol product derived from an aluminum salt is incorporated, wherein thesol product is formed through reaction between an aluminum salt and acompound, such as an inorganic alkaline compound which dissociates ahydroxide ion in an aqueous solution, or ammonia or an amine compoundwhich generates a free hydroxide group through hydration.

[0031] The sol product derived from an aluminum salt effectivelyenhances the polishing rate synergistically with the polishingaccelerator, modifies the viscosity of the entire composition andenhances dispersibility and re-dispersibility of alumina particles. Inaddition, the sol product enhances retention of alumina on a polishingpad to thereby reduce the amount of roll-off.

[0032] No particular limitation is imposed on the alumina that is usedin the present invention, and alumina of any crystal structure type suchas α, θ or γ may be used. Of these, α-alumina is preferred from theviewpoint of provision of a high polishing rate. Although no particularlimitation is imposed on the particle size of alumina, the averageparticle size is preferably 0.02-5 μm, more preferably 0.3-2 μm. In thepresent invention, the average particle size may be appropriatelydetermined in accordance with the target surface roughness to beattained.

[0033] The polishing composition used in the present inventionpreferably contains alumina in an amount of 1-30 mass %, more preferably3-20 mass %.

[0034] The sol product derived from an aluminum salt that is used in thepolishing composition of the present invention is obtained by adding,while mixing by means of a high-shear stirrer, at least one speciesselected from the group consisting of sodium hydroxide; potassiumhydroxide; ammonia; an organic amine compound, such as a C1-C10alkylamine (e.g., monomethylamine, dimethylamine, trimethylamine ormethyl-ethylamine) or a hydroxyalkylamine (e.g., triethanolamine); a(poly)amine chelate compound (ethylenediamine, diethylene-triamine orbipyridine); an aminocarboxylic acid (also called amino acid, such asglycine or glutamic acid); a (poly)amino-carboxylic acid chelatecompound (e.g., ethylenediamine-tetraacetic acid (EDTA),diethylenetriaminepentaacetic acid (DTOA), nitrilotriacetic acid (NTA),or iminodiacetic acid); and an aminophosphonic acid chelate compound,such as diethylene-triaminepentamethylenephosphonic acid oraminotrismethylene-phosphonic acid), to an aqueous solution containing,in the form of hydrate or anhydrate, at least one species selected fromamong inorganic acid aluminum salts, such as aluminum sulfate, aluminumchloride, aluminum nitrate, aluminum phosphate and aluminum borate; andorganic acid aluminum salts, such as aluminum acetate, aluminum lactateand aluminum stearate. Mixing an aluminum salt with a substance (e.g.,ammonia or amine) that readily generates a free hydroxide group uponreacting with water; a compound having a terminal hydroxide group; or acompound having a hydroxide group such as sodium hydroxide or potassiumhydroxide forms a chain-structure substance in which the componentcompounds are linked in a chain form.

[0035] The stirring step taken when obtaining the sol product derivedfrom an aluminum salt is performed using a high-shear stirrer that cangive high shearing force to the raw material mixed solution of thealuminum salt. This enables formation of a sol product suitable forpolishing a magnetic recording disk substrate.

[0036]FIG. 1 shows one example of a stirrer that performs high-shearstirring, in which a shield cylinder 5 equipped therein with a turbine 4is provided within a stirrer vessel 1 as supported by support rods 7 ina suspending manner.

[0037] In the stirrer having the above structure, when the turbine 4 isrotated at high speed via a turbine shaft 6, the raw material mixedsolution of the aluminum salt is stirred within the shield cylinder 5while being given a shearing force. The solution ascends in the form ofa convection current from an upper opening of the shield cylinder 5,forms a downward convection current 9 along the inside peripheral wallof the stirrer vessel in the presence of a commutation plate 3 providedin the vicinity of a liquid level 3, ascends toward a lower opening ofthe shield cylinder 5 at the lower side of the stirrer vessel, againundergoes high-shear stirring within the shield cylinder 5, and iscirculated by convection within the stirrer vessel 1. As a result, a solproduct is obtained.

[0038] The number of revolutions of the turbine 4 in the high-shearstirrer varies depending on the size of the stirrer vessel 1, shape ofthe turbine 4 and volume of the shield cylinder 5. When the stirrervessel 1 has a volume of 2-20 liters, the turbine is rotated at500-12,000 rpm, preferably 6000-10,000 rpm, more preferably 7,000-9,000rpm. When the volume of the stirrer vessel is twice, the number ofrevolutions of the turbine should preferably be about one second. Whenthe stirrer vessel 1 has a volume of 2-20 liters, the stirring time is15-60 minutes. It is necessary to increase the stirring time withincreasing volume of the vessel.

[0039] The present inventors have analyzed the structure of the solproduct through X-ray diffraction and NMR. The analysis revealed thatsol products according to the present invention; e.g., those producedfrom aluminum sulfate and ammonia; aluminum chloride and ammonia;aluminum sulfate and sodium hydroxide; and aluminum sulfate andtriethanolamine, exhibit no characteristic peak attributed topseudo-boehmite or a similar species and exhibit broad diffractionpatterns, whereas a sol product obtained from boehmite alumina withgluconic acid or aluminum nitrate exhibits a peak attributed topseudo-boehmite crystals. Thus, the structure of the sol products of thepresent invention is considered to be amorphous.

[0040] Although the above results reveal no clear network structure inthe sol products according to the present invention, it is consideredthat the structure is formed by bonding Al atoms with free hydroxidegroups to form aluminum hydroxide Al(OH)₃ of a bayerite structure, andfurther hydrating the aluminum hydroxide to thereby form amorphoushydrated alumina AL(OH)3.nH₂O sol.

[0041] The composition including the sol product of the presentinvention contains an aluminum salt in an amount of 0.01-5 mass %,preferably 0.05-2 mass %. When the amount is less than 0.01 mass %,desirable effects cannot be attained, whereas when the amount is inexcess of 5 mass %, the composition gels, and surface defects such aspits and nodules are formed.

[0042] In one mode of the present invention, an organic acid or aninorganic acid salt may be employed as the polishing accelerator. Theorganic acid may be at least one species selected from the groupconsisting of malonic acid, succinic acid, adipic acid, lactic acid,malic acid, citric acid, glycine, aspartic acid, tartaric acid, gluconicacid, peptogluconic acid, iminodiacetic acid and fumaric acid. Theinorganic acid salt may be at least one species selected from the groupconsisting of sodium sulfate, magnesium sulfate, nickel sulfate,aluminum sulfate, ammonium sulfate, nickel nitrate, aluminum nitrate,ammonium nitrate, ferric nitrate, aluminum chloride and nickelsulfamate.

[0043] The amount of the organic acid or the inorganic acid salt is0.01-10 mass %, preferably 0.1-2 mass %. When the amount is less than0.01 mass %, the effect of the polishing accelerator cannot be attained,whereas when the amount is in excess of 10 mass %, pits and nodules aregenerated to thereby deteriorate the quality of the polished surface. Inaddition, aggregation of alumina particles, which is undesirable for theliquid property, occurs.

[0044] In another mode of the present invention, as the polishingaccelerator, a combination of an organic acid and an organic orinorganic acid salt may be used. Similarly to the above case, theorganic acid may be at least one species selected from the groupconsisting of malonic acid, succinic acid, adipic acid, lactic acid,malic acid, citric acid, glycine, aspartic acid, tartaric acid, gluconicacid, peptogluconic acid, iminodiacetic acid and fumaric acid. Theorganic acid salt may be at least one species selected from the groupconsisting of a potassium salt, a sodium salt and an ammonium salt ofthe aforementioned organic acids.

[0045] Similarly to the above case, the inorganic acid salt may be atleast one species selected from the group consisting of sodium sulfate,magnesium sulfate, nickel sulfate, aluminum sulfate, ammonium sulfate,nickel nitrate, aluminum nitrate, ammonium nitrate, ferric nitrate,aluminum chloride and nickel sulfamate. In all combinations of theorganic acid and the organic acid salt and/or the inorganic acid salt,the total amount of the acids and salts is 0.01-10 mass % based on thetotal amount of the polishing composition, more preferably 0.1-2 mass %.Among these components, the amount of the organic acid must be adjustedto at least 0.003 mass %.

[0046] When the total amount of any combination in the polishingaccelerator is less than 0.01 mass %, the polishing acceleration effectbecomes poor, whereas when the amount is in excess of 10 mass %, theviscosity of a polishing composition solution increases excessively,aggregation of alumina particles undesirable for the liquid propertyoccurs, and pits and nodules are generated on the polished surface, tothereby disadvantageously lower the quality. In the case in which anorganic acid and an organic acid salt and/or an inorganic acid salt areused in combination, a combination of acids of the same species yieldsbetter polishing characteristics.

[0047] In addition to the aforementioned components, the polishingcomposition of the present invention may contain, as an additive and inaccordance with needs, any of alumina sol, a surfactant, a detergent, ananticorrosive agent, an antiseptic agent, a pH-controlling agent, athickener and a surface-modification agent such as cellulose, sulfamicacid or phosphoric acid.

[0048] The polishing composition of the present invention preferably hasa pH of 2-6.

[0049] A magnetic recording disk substrate can be obtained using thepolishing composition in the same manner as in the conventional methodthat comprises feeding the polishing composition between the surface ofa magnetic recording disk raw substrate to be polished and a polishingpad and rotating at least one of the raw substrate and the polishing padto permit the polishing composition to slide on the surface.

[0050] The present invention will next be described in more detail byway of examples, which should not be construed as limiting the inventionthereto.

EXAMPLES

[0051] Sixteen polishing compositions prepared to comprise differentcomponents are shown as Examples 1 to 16 in Table 1, and ten polishingcompositions prepared to comprise different components and not tocontain a sol product derived from an aluminum salt are shown asComparative Examples 1 to 10 in Table 2. Hereunder, the method ofpreparing these polishing compositions, formulation examples of solproducts derived from an aluminum salt, conditions for polishing arecording disk substrate, and a method for evaluating polishingcharacteristics of the substrate will be described.

[0052] (Preparation of Polishing Compositions)

[0053] Aluminum hydroxide was heated to about 1,200° C. in air in afiring furnace to obtain a-alumina. The thus-obtained a-alumina wascrushed and subjected to wet-classification, thereby producing threealumina samples having respective mean particle sizes of 0.6 μm, 0.7 μm,and 1.0 μm. In each Example or Comparative Example, the specificaluminum salt and the ammonia or other basic compound were mixed at thecompositional proportion shown in Table 1 or 2 to thereby prepare a solproduct derived from the aluminum salt under the stirring conditionsshown below. Formulation examples containing the respective sol productsare collectively shown below.

[0054] Subsequently, on the basis of the respective compositions shownin Table 1 or 2, water, alumina, a sol product derived from an aluminumsalt and a polishing accelerator were weighed, incorporated and mixed tothereby prepare the corresponding polishing composition samples.

[0055] (Formulation Examples for Producing a Sol Product Derived From anAluminum Salt)

[0056] Formulation examples (proportions by weight) for producing a solproduct derived from an aluminum salt are shown hereunder.

[0057] (1) Sol product derived from aluminum sulfate and ammonia:

[0058] water:aluminum sulfate (18 hydrate, employed hereinafter asaluminum sulfate):28% aqueous ammonia=20:5:3.6

[0059] (2) Sol product derived from aluminum sulfate andaminotris-methylenephosphonic acid (abbreviated as NTMP):

[0060] water:aluminum sulfate:NTMP=20:5:15

[0061] (3) Sol product derived from aluminum sulfate anddiethylenetriaminepentamethylenephosphonic acid (abbreviated as DTPMP):

[0062] water:aluminum sulfate:DTPMP=20:5:15

[0063] (4) Sol product derived from aluminum sulfate and triethanolamine(abbreviated as TEA):

[0064] water:aluminum sulfate:TEA=20:5:15

[0065] (5) Sol product derived from aluminum chloride and ammonia:

[0066] water:aluminum chloride (hexahydrate):28% aqueousammonia=20:5:3.6

[0067] (6) Sol product derived from aluminum nitrate and ammonia:

[0068] water:aluminum nitrate (nonahydrate, employed hereinafter asaluminum nitrate):28% ammonia water=20:5:3.6

[0069] (7) Sol product derived from aluminum nitrate andtriethanolamine:

[0070] water:aluminum nitrate:TEA=20:5:15

[0071] (8) Sol product derived from aluminum sulfate and sodiumhydroxide:

[0072] water:aluminum sulfate:50% sodium hydroxide=20:5:3

[0073] The amount of the sol product added to the composition (content)is defined as the total weight of the employed aluminum salt and thecompound for forming the sol product, the water content of eachcomponent being subtracted.

[0074] (Stirring Conditions for Producing a Sol Product)

[0075] The raw mixed solution prepared in advance was subjected tohigh-shear stirring using the high-shear stirrer with the structureshown in FIG. 1 to thereby obtain a sol product.

[0076] The stirrer used was T. K. Homoxer, MK-II model of M-type, aproduct of Japan Special Machine Chemical Industry Co., Ltd. Thestirring vessel had a volume of 2 liters, and the number of revolutionsof the turbine in the shield cylinder was 8,000 rpm.

[0077] (Polishing Conditions)

[0078] An aluminum disk having a size of 3.5 inches chemically platedwith Ni—P was employed as a workpiece to be polished. The polishing testand disk evaluation were carried out under the following conditions.

[0079] Polishing Test Conditions:

[0080] Polishing test machine: 9B double-sided polishing machine (aproduct of System Seiko)

[0081] Polishing pad: Politex DG

[0082] Number of revolutions of surface plate: upper surface plate 28rpm, lower surface plate 45 rpm, Sun gear 8 rpm

[0083] Feed rate of slurry: 100 ml/min.

[0084] Polishing time: 5 minutes

[0085] Operation pressure: 80 g/cm²

[0086] (Disk Evaluation Method)

[0087] Polishing rate: calculated from difference in weight before andafter polishing the disk

[0088] Quality of polished surface: pits, nodules and scratches on disksobserved under a microscope and counted; specifically pits and nodulescrosswise observed on both sides of five disks, and the number withinthe visual field (magnification: ×50) counted; and scratches crosswiseobserved on both sides of one disk, and the number within the visualfield (magnification: ×100) counted.

[0089] Amount of roll-off: measured by use of a surfcorder, SE-30D model(a product of Kosaka Kenkyujo).

[0090] To be specific, the circumferential portion of the surface of apolished rigid disk is traced by use of a surfcorder to draw a curve Sas shown in FIG. 2. A perpendicular line h is drawn along thecircumferential edge of the curve S. Points on the curve S which are at3,000 μm and 2,000 μm from the perpendicular line h towards the centerof the disk are assigned as A and B, respectively. On an extension ofthe straight line passing the points A and B, a point which is at 500 μmfrom the perpendicular line h is assigned as C. A perpendicular line kis drawn so as to pass the point C, and a point at which theperpendicular line k and the curve S are crossed is assigned as D. Thelength t between the points C and D is determined as the amount ofroll-off of the disk. TABLE 1-1 α-Alumina Sol Par- product Evaluation ofpolishing ticle Polishing accelerator derived Polish- size Acid saltratio from Al ing Surface defects D50 Amount Organic acid(organic/inorganic) salt rate Nod Pit Scr ROA Ex. μm % Type % Type % %μm/min No. No. No. Å 1 0.7 6 None 0 None 0 ALS/AM 0.74 0 4 2 800 1.0 20.7 6 Lac 0.5 Na lactate 1.0 ALS/AM 1.27 0 2 1 450 0.5 3 0.6 ditto Lac0.5 Na lactate 1.0 ALS/AM 0.96 0 2 2 700 0.5 4 1.0 ditto Mal 0.7 Namalate 0.2 ALS/AM 1.37 0 3 2 300 0.5 5 0.7 ditto Mal 0.7 Na malate 0.2ALS/AM 1.29 0 2 1 550 0.5 6 0.6 ditto Mal 0.7 Na malate 0.2 ALS/AM 1.020 2 1 650 0.5 7 0.7 ditto Mal 0.7 Na malate 0.2 ALS/AM 1.28 0 3 1 5001.0 8 0.7 ditto Mal 5.0 Na malate 4.0 ALS/AM 1.32 0 3 2 450 1.0

[0091] TABLE 1-2 α-Alumina Sol Par- product Evaluation of polishingticle Polishing accelerator derived Polish- size Acid salt ratio from Aling Surface defects D50 Amount Organic acid (organic/inorganic) saltrate Nod Pit Scr ROA Ex. μm % Type % Type % % μm/min No. No. No. Å 9 0.76 Mal 0.7 Na malate 0.2 ALS/TEA 1.30 0 2 2 550 0.5 10 0.7 ditto Mal 0.7Na malate 0.2 ALC/AM 1.32 0 2 2 500 0.5 11 0.7 ditto Mal 0.7 Na malate0.2 ALN/AM 1.31 0 3 2 500 0.5 12 0.7 ditto Glu 0.5 Na gluconate 0.5ALS/AM 1.17 0 1 1 600 0.5 13 0.7 ditto Mal 0.7 — — ALS/AM 1.24 0 4 2 5500.5 14 0.7 ditto — — Al nitrate 1.0 ALS/AM 1.26 0 3 2 450 0.5 15 0.7ditto Mal 0.7 Ni sulfate 0.3 ALS/AM 1.26 0 3 2 550 0.5 16 0.7 ditto Mal0.7 Na malate 0.2 ALS/AM 1.29 0 3 2 600 Al nitrate 0.2 0.5

[0092] TABLE 2 α-Alumina Sol Par- product Evaluation of polishing ticlePolishing accelerator derived Polish- size Acid salt ratio from Al ingSurface defects Comp. D50 Amount Organic acid (organic/inorganic) saltrate Nod Pit Scr ROA Ex. μm % Type % Type % % μm/min No. No. No. Å 1 0.76 None 0 None 0 None 0.56 M M M 2500 2 0.7 6 Lac 0.5 Na lactate 1.0 None1.18 0 6 4  800 3 1.0 ditto Mal 0.7 Na malate 0.2 None 1.27 0 8 5 1000 40.7 ditto Mal 0.7 Na malate 0.2 None 1.18 0 7 3 1400 5 0.6 ditto Mal 0.7Na malate 0.2 None 0.93 1 9 3 1900 6 0.7 ditto Glu 0.5 Na gluconate 0.5None 1.08 0 6 4 1600 7 0.7 ditto Mal 0.7 — — None 1.15 1 8 4 1800 8 0.7ditto — — Al nitrate 1.0 None 1.19 0 10 5 1250 9 0.7 ditto Mal 0.7 Nisulfate 0.3 None 1.17 1 8 4 1700 10 0.7 ditto Mal 0.7 Na malate 0.2 None1.20 1 10 4 1750 Al nitrate 0.2

[0093] As described hereinabove, the polishing composition of thepresent invention comprising at least water, alumina and a sol productderived from an aluminum salt, and optionally a polishing acceleratorattains a high polishing speed, a small amount of roll-off, and ahigh-quality mirror-finished surface free of surface defects that isoptimal as the surface of a magnetic recording disk substrate.

[0094] As is clear from comparison between Tables 1 and 2, addition ofthe sol product reduces the amount of roll-off to one third or smaller,provides a polished surface of good surface properties with few pits,nodules and scratches, and attains a high polishing rate. By adding apolishing accelerator to the polishing composition of the presentinvention, the polishing rate can be further enhanced, attaining aremarkable effect, with the amount of roll-off enhanced to around 500 Å.

What is claimed is:
 1. A polishing composition comprising at leastwater, alumina and a sol product derived from an aluminum salt.
 2. Apolishing composition according to claim 1, further comprising apolishing accelerator.
 3. A polishing composition according to claim 2,wherein the polishing accelerator is at least one species selected fromthe group consisting of organic acids, inorganic acids and saltsthereof.
 4. A polishing composition according to claim 1, wherein thesol product is a mixture of an aluminum salt with at least one speciesselected from the group consisting of sodium hydroxide, potassiumhydroxide, ammonia, organic amine compounds, amine chelate compounds,aminocarboxylic acids, aminocarboxylic acid chelate compounds andaminophosphonic acid chelate compounds.
 5. A polishing compositionaccording to claim 2, wherein the sol product is a mixture of analuminum salt with at least one species selected from the groupconsisting of sodium hydroxide, potassium hydroxide, ammonia, organicamine compounds, amine chelate compounds, aminocarboxylic acids,aminocarboxylic acid chelate compounds and aminophosphonic acid chelatecompounds.
 6. A polishing composition according to claim 1, wherein thesol product is a mixture of at least one species selected from amonghydrates and anhydrates of aluminum salts including inorganic acidaluminum salts that include aluminum sulfate, aluminum chloride,aluminum nitrate, aluminum phosphate and aluminum borate, and organicacid aluminum salts that include aluminum acetate, aluminum lactate andaluminum stearate with at least one species selected from among sodiumhydroxide, potassium hydroxide, ammonia, organic amine compounds, aminechelate compounds, aminocarboxylic acids, aminocarboxylic acid chelatecompounds and amino-phosphonic acid chelate compounds.
 7. A polishingcomposition according to claim 2, wherein the sol product is a mixtureof at least one species selected from among hydrates and anhydrates ofaluminum salts including inorganic acid aluminum salts that includealuminum sulfate, aluminum chloride, aluminum nitrate, aluminumphosphate and aluminum borate, and organic acid aluminum salts thatinclude aluminum acetate, aluminum lactate and aluminum stearate with atleast one species selected from among sodium hydroxide, potassiumhydroxide, ammonia, organic amine compounds, amine chelate compounds,aminocarboxylic acids, aminocarboxylic acid chelate compounds andamino-phosphonic acid chelate compounds.
 8. A polishing compositionaccording to claim 1, wherein the sol product is a mixture of at leastone aluminum salt selected from the group consisting of aluminumsulfate, aluminum chloride and aluminum nitrate with at least onecompound selected from the group consisting of sodium hydroxide,potassium hydroxide, ammonia, triethanolamine andaminotrismethylenephosphonic acid.
 9. A polishing composition accordingto claim 2, wherein the sol product is a mixture of at least onealuminum salt selected from the group consisting of aluminum sulfate,aluminum chloride and aluminum nitrate with at least one compoundselected from the group consisting of sodium hydroxide, potassiumhydroxide, ammonia, triethanolamine and aminotrismethylenephosphonicacid.
 10. A polishing composition according to claim 2, wherein thepolishing accelerator is contained in an amount of 0.01-10 mass %.
 11. Apolishing composition according to claim 1, wherein the sol product iscontained in an amount of 0.01-5 mass %.
 12. A polishing compositionaccording to claim 2, wherein the sol product is contained in an amountof 0.01-5 mass %.
 13. A method of producing a sol product derived froman aluminum salt, which comprises mixing, by means of a stirrer, analuminum salt with at least one species selected from the groupconsisting of sodium hydroxide, potassium hydroxide, ammonia, organicamine compounds, amine chelate compounds, aminocarboxylic acids,aminocarboxylic acid chelate compounds and aminophosphonic acid chelatecompounds.
 14. A method of producing a sol product according to claim13, wherein the aluminum salt is at least one species selected from thegroup consisting of aluminum sulfate, aluminum chloride and aluminumnitrate.
 15. A method of producing a sol product according to claim 13,wherein the stirrer is a high-shear stirrer.
 16. A method of producing asol product according to claim 14, wherein the stirrer is a high-shearstirrer.
 17. A method of producing a magnetic recording disk substrate,which comprises rotating at least one of a magnetic recording disk rawsubstrate and a polishing pad while the polishing composition as recitedin claim 1 is fed into a space between the substrate and the pad.
 18. Amethod of producing a magnetic recording disk substrate, which comprisesrotating at least one of a magnetic recording disk raw substrate and apolishing pad while the polishing composition as recited in claim 2 isfed into a space between the substrate and the pad.
 19. A method ofproducing a magnetic recording disk substrate according to claim 17,wherein the magnetic recording disk raw substrate is an aluminummagnetic recording disk substrate that is chemically plated with Ni—P.20. A method of producing a magnetic recording disk substrate accordingto claim 18, wherein the magnetic recording disk raw substrate is analuminum magnetic recording disk substrate that is chemically platedwith Ni—P.